1. Field of the Invention
The present invention relates to a toner for electrophotography. More particularly, the invention relates to a toner for electrophotography which has rheological properties including excellent fixing properties and which causes less contamination when the toner is fixed.
2. Description of the Related Art
In imaging apparatuses, such as electrophotographic devices or electrostatic recording devices, an electrostatic latent image is formed on a uniformly charged photoreceptor through an image exposing process, a toner is attached to the electrostatic latent image to form a tone image, and then the tone image is transferred to a transfer material, such as a transfer sheet. Subsequently, the tone image which is not yet fixed is fixed on the transfer material by a heating process, a compressing process, a solvent evaporating process, or the like. In the fixing process, in general, the transfer material to which the tone image is transferred passes through a dwell roll and a pressing roll, and thus, the toner is heated and compressed to be melted on the transfer material. At this time, the toner that is to be fixed is fixed on the transfer material according to fixing conditions and a stable image is formed.
In a heat fixing method using a heating roller or film, excellent heat efficiency is required to hot-melt adhere the tone image to a dwell sheet when the surface of the heating roller or film contacts the toner image on the dwell sheet. In addition, a high thermal capacity of a heat fixing unit is also required to prevent fixing failure due to the passage of the dwell sheet and low-temperature fixing. Accordingly, there is a need to improve the properties of a toner, specifically the low-temperature fixing properties of a toner in order to realize low electric power consumption with excellent fixing properties.
For example, in a fixing process of a pressure-heat fixing method, the surface of a high-temperature dwell roller and the tone image are contacted with each other in a melted phase under pressure, so that the toner is transferred and attached to the surface of the high-temperature dwell roller, and then the toner is transferred again to the subsequent dwell sheet so that the dwell sheet is contaminated, which is called an offset phenomenon. The offset phenomenon is heavily affected by fixing speed and temperature. In general, when the fixing speed is low, the surface temperature of a dwell roller is set to low. On the other hand, when the fixing speed is high, the surface temperature of a dwell roller is set to high. Such adjustments are required since there is a need to provide a constant heat capacity to the toner image in order to fix the tone image independently from the fixing speed.
The problem can be solved by increasing the fixing temperature when the fixing speed is high to easily fix the toner. In this method, the temperature of the heating roller can be decreased and high-temperature offset phenomenon at the most upper portion of the toner layer can be prevented. However, the method can cause several problems. For example, a winding offset phenomenon wherein a dwell sheet may surround the dwell roller when a very high shear force is applied to the toner layer may occur, and a separation unit that separates the dwell sheet from the dwell roller may leave an impression in a fixed image.
Korean Registered Patent No. 138,583, Korean Laid-open Patent Publication No. 2001-083034, and Korean Laid-open Patent Publication No. 1999-063467 disclose electrophotostatic toners having rheological properties, but do not disclose a way of obtaining excellent fixing properties and preventing the offset phenomenon at the same time. Accordingly, with respect to the heat transfer phenomenon and rheological properties of the toner, there is a need to develop a technique capable of improving fixing properties of the toner according to heat and pressure and preventing contamination.